The present invention relates, in general, to probes, and more particularly, to probes positioned using piezoelectric transducers.
In general, the resistivity of a grown or diffused layer in a semiconductor, the resistivity of a metallic film, or the resistivity of an electrically conductive film, is an important parameter which represents the electrical characteristics of these materials. In particular, spreading resistance is often used to measure thickness of diffused or epitaxial layers, and for establishing the impurity profile for these structures. Spreading resistance is the resistance associated with the divergence of the current lines which emanate when a small-tipped electrical probe is placed onto a semiconducting or conducting surface.
Commercially available spreading resistance probe (SRP) apparatus usually use two probes. The two probes are carefully machined to provide a controlled shape and size. The two probes are mounted in a fixed relationship to each other, typically about twenty micrometers apart and the probes are in the same plane. Because non-reproducible contact resistance between the probes and the sample causes measurement error, the probes are usually forced into the sample surface with a gravity load to provide a reproducible ohmic contact between the probes and the sample surface.
A known current is applied between the two probes, and the voltage drop is measured across these probes to obtain a spreading resistance. A major use of SRPs is to determine doping profiles of diffused layers. This is accomplished by angle lapping a sample to provide a beveled surface and then making spreading resistance measurements along the length of the bevelled surface. Previous SRP apparatus use pneumatic drive means to position the probes in relation to the sample and to move the probes along the length of the bevelled surface.
One disadvantage of previous SRPs is that spacing between the probes is difficult to control, and is quite large by modern device standards. Many semiconductor devices have diffused structures which are only a few microns or less in size. Thus, an SRP with probe spacing of twenty microns could not be used. Prior SRPs do not provide any method to accurately monitor the probe tip spacing during calibration and testing. Moreover, because the probes are in a fixed position with respect to each other, it is very difficult to make spreading resistance measurements in two dimensions. Because lateral diffusion in modern small geometry devices is as important to device performance as vertical diffusion, it is desirable to be able to accurately measure spreading resistance in two dimensions.
Another disadvantage of previous SRPs is that because the probes are forced into the sample surface with a fixed and significant force, the surface of the sample is damaged with unpredictable effects on measurement accuracy. Because the probes penetrate 0.005-0.01 micrometer into the sample surface, only a few measurements can be taken on a junction which is on the order of 0.1 micrometer deep. Also, the large force damages fine-tipped probes, requiring the use of large diameter probes. Large diameter probes, as set out above, require correspondingly large spacing between the probes.
An additional disadvantage of previous SRP apparatus is that the pneumatic drives used to position the probes provide limited precision in positioning the probes resulting in limited resolution and accuracy of the impurity profiles calculated from the spreading resistance measurements. Improved depth resolution results from minimal incremental position changes when moving the probes along the length of the bevelled surface. However, conventional pneumatic drives provide only relatively large incremental position changes and correspondingly limited depth resolution.
Accordingly, it is desirable to have an SRP apparatus that has probes which can be multi-dimensionally positioned with high-resolution, that minimizes penetration depth into a sample surface, that is compact and does not produce vibrations.